1. Field of the Invention
The present invention relates to an equalizer, and more particularly, to a continuous-time adaptive equalizer.
2. Description of the Related Art
As is well known in the art, when a signal is transferred via a transmission line, the signal decays during the transmission process. This means that for transmission lines having a great length, the signal decay is more obvious. Therefore, when the signal arrives at the destination (for example, a receiver), the signal received by the receiver may contain distortions.
In order to solve the above-mentioned problem, an equalizer is often established logically in front of (i.e., before) the receiver such that the original signal can be recovered from the decayed signal. This allows the receiver to correctly process the received signal.
For example, in the data transmission of a USB interface, a PCI-E interface of a computer system, or the data transmission of an HDMI interface of an LCD, high-frequency portions of the signal often decay in the transmission process. Therefore, the aforementioned equalizer is often established at the receiving ends of the above-mentioned interfaces to provide an appropriate gain for the high-frequency portions of the signal such that the original signal can be recovered from the decayed signal. In this way, said computer system or said LCD can process the recovered signal.
In “ISSCC2005/SESSION18/HIGH-SPEED INTERCONNECTS AND BUILDING BLOCKS/18.1 (A 10 Gb/s CMOS Adaptive Equalizer for Backplane Applications)” published by Srikanth Gandi, Jri Lee, Daishi Takeuchi, and Brhzad Razavi, a continuous-time adaptive equalizer is disclosed. Please refer to FIG. 1. FIG. 1 is a block diagram of a conventional continuous-time adaptive equalizer 100. As shown in FIG. 1, the continuous-time adaptive equalizer 100 comprises an active high-pass filter 110, a slicer 120, a boost control module 130, a swing control module 140, and a buffer 150.
The active high-pass filter 110 is utilized to perform the above-mentioned amplifying (signal recovery) operation. In other words, the active high-pass filter 110 performs the amplifying operation on the high-frequency portions of the received data signal Din to output a processed data signal Dout. The processed data signal Dout is then buffered by the buffer 150, and is further outputted to a following receiver (not shown in FIG. 1).
However, when the equalizer 100 receives the data signal Din, the equalizer 100 cannot predict the condition of the data signal Din. For example, the equalizer 100 cannot determine the degree of signal decay. Therefore, the equalizer 100 requires an adjusting mechanism to dynamically adjust the filtering frequency band of the active high-pass filter according to the properties of the data signal Din.
In this case, the slicer 120 and the boost control module 130 are utilized as the above-mentioned adjusting mechanism. As shown in FIG. 1, the slicer 120 and the boost control module 130 form a feedback loop to control the filtering frequency band of the active high-pass filter 110.
The slicer 120 converts the signal at node A (Dout) into a square wave. The boost control module 140 outputs a feedback signal to the active high-pass filter 110 according to the difference between the signals at the node A and node B such that the filtering frequency band (i.e., the frequency response) of the active high-pass filter 110 can gradually approach a desired frequency band according to the feedback signal. In this way, the data signal Dout eventually may resemble the originally transmitted data signal.
However, if only the boost control loop is utilized, the accuracy of the data signal Dout cannot be guaranteed. Please note, the equalizer 100 cannot determine the amplitude of the data signal Dout when receiving the data signal Dout. It is apparent that if the square wave outputted from the slicer 120 does not correspond to the signal Dout at the node A, then the boost control module 130 may lock the entire equalizer 100 on an incorrect operational point. This will cause the active high-pass filter 110 have incorrect frequency responses such that the data signal Dout cannot be recovered as the original data signal.
Therefore, the swing control module 140 is established to solve the above-mentioned problem. In this case, the slicer 120 and the swing control module 140 form another feedback loop. The additional feedback loop is utilized to control the slicer 120 to output a square wave having a desired amplitude. In other words, the swing control module 140 outputs another feedback signal according to the amplitude differences between the signals at node A and node B such that the slicer 120 is controlled to output the square wave having the same amplitude as the signal at the node A.
Utilizing these two above-mentioned feedback loops, the equalizer 100 can ensure that the outputted data signal Dout is very close to corresponding to the original data signal. This results in an improvement to the signal transmission quality.
Unfortunately, the above-mentioned equalizer 100 has disadvantages. As shown in FIG. 1, two feedback loops lie in the same signal route. As is well known, the, two feedback loops cannot work simultaneously. In fact, if the two feedback loops were simultaneously in operation then the stability of the entire equalizer 100 may be reduced. For example, the two feedback loops may introduce an oscillation, and thus the stability of the equalizer 100 is reduced by said oscillation. Therefore, in the actual application, in order to increase the stability of the equalizer 100, the operational speed (i.e., the frequency band) of the swing control module 140 (the swing control loop) must be greater than that of the boost control module 130. In other words, the equalizer 100 should firstly use the swing control module 140 to make the amplitudes of the signals at the node A and node B equal. Secondly, after the amplitudes of the signals at the node A and node B are adjusted, the equalizer 100 is switched such that the boost control module 130 becomes active and starts operation. In this way, the boost control module 130 can start to perform the above-mentioned feedback control to adjust the high frequency swing of the data signal Dout such that the high frequency swing of the data signal Dout can be outputted correctly.
To speak more simply, the circuit designer must evaluate and analyze the stability of the equalizer 100 and thereafter implement a more complex design to ensure that the equalizer 100 operates correctly. If the equalizer 100 is poorly designed, (for example, the operating time of the swing control module 140 is insufficient to make the amplitudes of the signals at the node A and node B equal to one another), the boost control module 130 cannot correctly perform the feedback control such that the active high-pass filter 130 has incorrect frequency responses and the original data cannot be recovered.
Furthermore, if the frequency bandwidth of the boost control module 130 is lower than the swing control module 140, larger capacities must be utilized in the boost control module 140. This increases the space requirements of the entire circuit and thereby increases the power consumption of the equalizer 100.